An omni-base station is a base station that is configured to use an omni-antenna, and a sector base station is configured to use multiple (two or more) sector antennas. FIG. 1A shows a single cell area for a base station (BS) with an omni-antenna. An omni-antenna radiates 360 degrees to provide coverage over the entire cell area. FIG. 1B shows single cell area for a base station (BS) with three sector antennas. A three sector base station is a common sector configuration, but more or less sectors could be used. In this case, the cell area is divided into thirds, with each sector antenna having a narrower beam (as compared to an omni-antenna) that radiates to provide coverage over its sector area of approximately 120 degrees.
A base station antenna is often mounted in an elevated location, such as on a tower, a pole, on the top or sides of buildings, etc., to enhance coverage and provide better possibilities for direct radio signal propagation paths. FIG. 2A shows a base station unit 14 located at the base of a tower 12. An antenna 10 is mounted on the top of the tower 12 and is coupled via a feeder cable 16, typically a coaxial cable or the like, to the base station transceiver. The received signal suffers signal losses traversing the feeder 16, and the taller the tower 12, the longer the feeder, and the greater the loss. In order to offset such signal losses in the feeder, a tower-mounted amplifier (TMA) may be used to amplify the received signal before it is sent over the feeder to the base station unit. FIG. 2B shows a TMA 18 mounted at the top of the tower 12 near antenna 10. A tower mounted unit is sometimes called a mast head amplifier. The term tower mounted amplifier (TMA) is used generically herein to include any device that performs this pre-feeder amplification function.
FIG. 3 shows a simplified block diagram of an omni-base station 20. The antenna 10 is coupled to a duplex filter 21 in the TMA 18 which includes a receive (Rx) filter 22 and a transmit (Tx) filter 24. The duplex filter makes it possible to send and receive on the same antenna by separating the Tx and Rx signals from each other. The transmit filter 24 is coupled directly to the feeder 16, and the receive filter 22 is coupled to the feeder 16 via a low noise amplifier (LNA) 26. The feeder 16 couples to the base station 14 which also includes a duplex filter 28 having a receive filter (Rx) 30 and a transmit (Tx) filter 32. The transmit filter 32 is coupled to the transceiver 36, and the receive filter 30 is coupled to the transceiver 36 via a low noise amplifier 34.
Antenna diversity may be used in order to improve reception (or transmission) of transmitted radio signals. There are many kinds of diversity, such as time diversity, space diversity, and combinations thereof. A promising diversity scheme uses time/space coded signals and is referred to as Multiple Input Multiple Output diversity (MIMO). Space diversity reduces the effects of fading received radio signals. An antenna diversity systems comprises at least two antennas arranged at a distance from each other. In the case of receive diversity, the received signal is received on the two or more antennas. The Rx signals from the diversity antennas are subjected to diversity processing in order to obtain an enhanced signal. Diversity processing may, for example, include selecting the antenna signal which is strongest, or adding the signals and further processing the resulting signal. In transmitter diversity, the transmit TX signal is transmitted on the two or more transmit antennas to which the transmitter is connected. Antennas of a diversity arrangement are called diversity antennas. In diversity arrangements, a feeder and its associated antenna may be referred to as a diversity branch or simply branch.
FIG. 4 shows an example of an omni-base station 14 with diversity. Two diversity antennas 10a and 10b are coupled to corresponding TMAs 18a and 18b. Each TMA is coupled by a corresponding feeder 16a and 16b to a corresponding duplex filter and low noise amplifier unit 42a and 42b in the base station 14. The two duplex filter and LNA units 42a and 42b are coupled to a single transceiver 36.
In contrast to the single transceiver used in the omni-base station, a sector base station such as that shown at 50 in FIG. 5 has a separate transceiver for each sector. Three sectors are supported with each sector having its own antenna 101, 102, and 103. Each of the antennas 101, 102, and 103 is coupled to a corresponding sector TMA 181, 182, and 183. Three feeders 161, 162, and 163 couple respective TMAs 181, 182, and 183 to corresponding base station units 141, 142, and 143. Each of the base station units 141, 142, and 143 has a corresponding duplex filter and low noise amplifier unit 421, 422, and 423. A sector base station provides more coverage than an omni-base station but at higher monetary and power costs.
Although omni-base stations are less complex and less expensive than sector base stations, they also provide less coverage, and therefore, an operator must install more omni-base stations to cover a particular geographic area than if sector base stations were installed. In response, multi-sector omni-base stations were introduced where an omni-base station is coupled to a multi-sector antenna system. In fact, in an example where a three sector antenna system is used with an omni-base station, the three sector antenna system adds approximately 7-8 dB of signal gain. Another benefit of a multi-sector omni-base station is the ability to “tilt”, e.g., downtilt, one or more of the sector antennas. Tilting is not an option for omni antennas.
An example of a three sector base station 60 is shown in FIG. 6A. Three sectors are supported with each sector having its own antenna 101, 102, and 103. Each of the antennas 101, 102, and 103 is coupled to a corresponding sector TMA 181, 182, and 183. Three feeders 161, 162, and 163 couple respective TMAs 181, 182, and 183 to the base station 14. The base station 14 includes three duplex filter and low noise amplifier units labeled generally at 42 coupled to three transceivers 36. But because feeder cables, duplex filters, and transceivers are expensive, (even more so when diversity is used in each sector), a splitter/combiner is used so that only one feeder is necessary. FIG. 6B shows how the received signals from the three sectors 1, 2, and 3 are combined together in a splitter/combiner 44 onto one feeder cable 16. In the transmit direction, the transmit signal is split into three identical signals (at lower power) and provided to each sector's TMA.
But the feeder cost savings attained by using a splitter/combiner is offset by the substantial power lost in the combiner. Indeed, in the three sector example mentioned above, the 7-8 dB gain achieved by using a three sector antenna system is offset by 5 dB lost in the combiner. That loss is attributable to the interference between the three sector receive signals caused when they are combined in the combiner. That frequency overlap interference significantly reduces the signal-to-noise ratio of the sector signals received in the base station transceiver. The power is split to three different sectors in the splitter for the downlink transmission at 5 dB (i.e., one third) less power for each sector. One approach available to deal with the downlink transmission loss is to simply increase the base station transmit power. But substantially increasing the mobile station transmission power levels across the board is not an option in the uplink because transmit power of mobile stations generally must be tightly controlled and limited.